The typical braking for automobile is realized by a contacting friction between a brake shoe and a brake drum. The frequently contacting friction exists between the brake shoe and the brake drum in the period of automobile running, and the clearance of both brake shoe and brake drum will become larger due to erosion, which has influence on the safety of the brake device during the vehicle operation. Usually there is an adjuster which can automatically adjust the brake clearance between the brake shoe and the brake drum in existing brake device of automobile. This adjuster can compensate the abrasion losses of the clearance in the brake shoe and the brake drum and then maintain the said clearance within in a constant value thus to improve the safety of working condition for the brake device of automobile.
“An automatic slack adjuster for automobile” is published under the Chinese Patent No. 200710055529.3, which consists of a casing in which a worm gear and a worm shaft are installed; on one end of the worm there is a set of coil spring, a spring seat and a spring retainer; a cam overrun clutch is connected with a socket on the worm shaft; the driven screw gear thereof is engaged with a driving screw gear of a clearance control device; the clearance control device comprises a planetary gear, a gear shaft, a driving screw gear; there is a damp device including an adjusting screw, spring and steel balls disposed on the left end of the driving screw gear; in the worm gear hole there is a control arm unit installed coaxially with the worm gear, the control arm unit comprises a sun gear coaxially installed in the worm gear hole, a tooth shaped socket, a spring retainer and a control arm, the sun gear is engaged with the planetary gear.
The driving screw gear is engaged with the driven screw gear of a cam overrun clutch; the screwed teeth is on the left end of the driving screw gear and three protruding sector blocks, each central angle being α, is on the right end; a straight gear engaged with the sun gear is installed on the right end of the planetary gear, there are three sector grooves at the left end of the planetary gear, each central angle is β. When three protruding sector blocks on the driving gear inserts into the three grooves on the planetary gear, a sector space α−β=γ appears between the protruding sector blocks and grooves. This angle is a racing angle of the planetary gear during normal braking of the brake system, which is a pre-set clearance by the clearance control device; Knowing from the above explanation, γ is a racing angle of the planetary gear during normal braking of the brake system, it is also a fixed value. A fixed racing angle can only be applied for certain one or several brake systems, if it is inapplicable, then it should be redesigned to define the angle γ, which, therefore, causes poor universality. In order to maintain the angle γ within a given range, a rather high production precision for manufacturing of the driving gear and the planetary gear is needed, which increase the cost of manufacturing and reduces the applicable field.